The present invention is an improved control system for a hydrodynamic-mechanical transmission in an automobile which includes one or more hydraulically actuatable gear shift elements. In such transmissions, the drive select lever is coupled to a manual control valve, which delivers a source of first pressurized fluid to the gear select element when the gear select lever is moved to automatic forward drive.
During certain operating states of the vehicle, for example during idling, it is undesirable that the transmission transmit the driving force of the engine to the driving wheels. In commonly owned U.S. Pat. No. 4,331,045, which is incorporated herein by reference, the otherwise engaged gear select elements are selectively disengaged during certain engine operating states by interposing a main control valve in the pressure medium line between the manual control valve and the gear select element. The main control valve may be dependent upon the position of the accelerator pedal to cause disengagement of the gear select elements. The valve both interrupts the delivery of the pressurized fluid from the manual control valve, and at the same time purges the pressure chamber actuating the gear select element, at least when the vehicle is at a stand still, the accelerator pedal is released, and the drive select lever is in the drive position D. Under such conditions, the fluid pressure on the gear select element associated with the forward drive gears is interrupted at least while the accelerator pedal remains released, or provided certain other operating states of the vehicle are not present. For example, a speed-dependent signal of the vehicle can override the interruption of the fluid pressure so that the engine remains coupled through the transmission to the driving wheels when the vehicle is moving.
In accordance with the arrangement, the main control valve prevents the transmission of a creeping moment when the engine idles and a forward gear is engaged (gear select position D), which also prevents vibration of the vehicle caused by transmitting the engine vibrations.
This control system is also suitable for disconnecting the positive connection between the engine and drive gears, for example when the accelerator pedal is released, during the vehicle travel. As such, the vehicle is in an overrun or deceleration mode, that is the wheels drive the engine. When the transmission is disconnected, however, the engine can run at idle. The braking effect of the engine is thereby suspended so that the vehicle, by way of its kinetic energy, travels over longer distances to reduce fuel consumption.
In some hydrodynamic-mechanical transmissions, a by-pass clutch, which is in itself known, is provided to reduce the losses present in a torque converter by by-passing the torque converter during certain vehicle operating states, in particular at higher transmission gears. When the by-pass clutch is actuated, the engine output shaft is connected directly to the mechanical gear train of the transmission, by-passing the hydrodynamic torque converter. However, during vehicle operation states in which the torque converter is by-passed, and there is a rigid rather than fluid coupling of the engine and transmission caused thereby, the engagement and disengagement of the gear control element by the main control valve, for example responsive to the release of the accelerator pedal during vehicle travel, is accompanied by pronounced shifting jerks.